二分查找（递归）

#include<stdio.h>

int BSearch(int a[],int low,int high,int x){
    if(low > high) return -1;
    int mid = (low + high )/ 2;
    if(a[mid] == x) return mid;
    else if(a[mid]<x) return BSearch(a,mid+1,high,x);
    else return BSearch(a,low,mid-1,x);
}

int main(){
    int a[] = {2, 3, 4, 10,40,55,61,72,73,88,90,95,100};
    int x;
    int n = sizeof(a) / sizeof(a[0]);
    scanf("%d",&x);
    int res = BSearch(a,0,n-1,x);
    if(res == -1){
        printf("不存在此元素");
    }
    else
    printf("%d %d",res,a[res]);
}

非递归版

#include<stdio.h>

int BSearch(int a[],int n,int x){
    int low = 0;
    int high = n;
    while(low<=high){
        int mid = (low+high)/2;
        if(a[mid]==x) return mid;
        else if(a[mid]<x) low = mid+1;
        else high = mid -1;
    }
    return -1;
}

int main(){
    int a[] = {2,3,4,10,40,55,61,72,73,88,90,95,100};
    int x;
    int n = sizeof(a) / sizeof(a[0]);
    scanf("%d",&x);
    int res = BSearch(a,n-1,x);
    if(res == -1){
        printf("不存在此元素");
    }
    else
    printf("%d %d ",res,a[res]);
}